Electrochemical sensors for the measurement of carbon monoxide in a gas sample are well known. Working, counter and reference electrodes are connected through a potentiostat circuit, which maintains a bias potential between the working and reference electrodes. The working electrode catalyses the oxidation of carbon monoxide. For a carbon monoxide sensor, the following reaction is catalysed at a platinum working electrode:2CO+2H2O→2CO2+4H++4e−
A counterbalancing reduction takes place at the counter electrode. This is typically the reduction of oxygen:O2+4H++4e−→2H2O
but may also be the formation of hydrogen:4H++4e−→2H2
The reference electrode assists in regulating the electrochemical reaction at the working electrode by providing a fixed potential.
As the number of electrons released at the working electrode, and so the working electrode current, is proportional to the amount of carbon monoxide that is oxidised, the working electrode current forms a signal which is proportional to the concentration of carbon monoxide in the gas sample.
For the proportional relationship between current and carbon monoxide concentration to apply, the sensor needs to be configured so that the reaction of carbon monoxide at the working electrode is controlled by the diffusion of the carbon monoxide to the working electrode surface. In order that the output current be diffusion controlled, two important conditions are (a) that there is a means for restricting the diffusion of the gas sample to the working electrode, typically in the form of a so-called capillary hole in the sensor casing; and (b) that an appropriate potential difference be applied between the working electrode and the reference electrode. Conveniently, setting a zero potential difference between working and reference electrodes is within the diffusion limited region, under standard conditions, although bias potentials are known to have been used in some carbon monoxide sensors.
In many of the circumstances in which carbon monoxide sensors are used, there may be other gases present which can also create a working electrode current indistinguishable from that due to carbon monoxide oxidation. These gases are known as interferents. There are several mechanisms by which an interferent can act. A gas may act as an interferent simply because it can also react at the working electrode at the bias potential in use. Alternatively, the interferent may react at the reference and/or counter electrodes, and may even be produced as a by-product of the function of the sensor itself; for example, hydrogen molecules may be evolved at the counter electrode under some conditions. Common interferents include hydrogen, alkenes and dihydrogen sulfide. The present invention primarily considers hydrogen interference, although the principles apply to other interferents. Interference can be quantified in the form of the cross sensitivity, defined below.
An important use for carbon monoxide sensors is in flue gas monitoring for monitoring completeness of combustion. In such circumstances, interference by hydrogen is common. Sensors for use in flue gas monitoring therefore need to have low hydrogen cross sensitivity. One relevant standard for flue gas monitoring sensors is the German Standard for the self-test of carbon monoxide measuring instruments for gas fixed appliances, released Aug. 5, 1988 (TÜV standard). The performance requirements dictated by TÜV for a carbon monoxide sensor include the following:                1. The sensors must respond linearly in the range 0 to 2000 ppm carbon monoxide from 10 to 30° C. with a maximum permissible error of ±5%.        2. The total error including hydrogen response must not exceed ±5% of the absolute output in a gas sample comprising 800 ppm hydrogen and 950 ppm carbon monoxide.        3. This performance must be achieved at 10° C., 20° C. and 30° C., in low oxygen environments.        
It can therefore be seen that multiple parameters must be considered when designing an appropriate electrochemical sensor. The combination of a wide linear range and a low hydrogen cross sensitivity, alongside other commercial considerations such as cost and compatibility with existing equipment present a design challenge.
There have been many different approaches to solving the problem of providing a carbon monoxide sensor with a low hydrogen cross sensitivity. Of these, the simplest method of improving hydrogen cross sensitivity, as is well known to those skilled in the art, is to reduce the amount of catalyst on the working electrode.
The reason for this is also well known. As discussed above, carbon monoxide sensors are operated in the diffusion controlled region, with respect to carbon monoxide, and so changing the amount of catalyst will not affect the sensor's response to carbon monoxide. However, the response to hydrogen is controlled by the rate at which the catalyst oxidises the hydrogen. Therefore, the signal due to hydrogen is a function of the number of hydrogen binding sites; ie, the amount of catalyst present. Therefore, the hydrogen signal decreases relative to the carbon monoxide signal as the amount of catalyst is reduced; ie the cross sensitivity decreases as the amount of catalyst is reduced.
Unfortunately, reducing the amount of catalyst on a conventional platinum black working electrode to a point where hydrogen cross sensitivity is below 5% means that there is so little catalyst present that the linear range of the sensor is very poor. As a result of this consequence (and the cost of platinum black) carbon monoxide sensors are sometimes designed to use the smallest amount of catalyst possible.
One method which has been used to mitigate hydrogen cross sensitivity is described in EP 0126623 (City Technology Ltd). This is a four electrode system in which two of these electrodes are working electrodes arranged in series. The first working electrode is set to a potential where it will oxidise both hydrogen and carbon monoxide; the second working electrode is not gas specific but provides an accurate reading of hydrogen remaining after removal of all of the carbon monoxide. A commercial limitation of this type of system is that the circuitry required to drive the four electrodes is different to that used for the conventional three electrode systems sold for testing most gases. Secondly, the use of four electrodes increases the difficulty of calibration and such sensors are prone to drift due to the non-equivalent changes of catalytic activity in the two working electrodes, reducing accuracy and reliability.
It is also known to provide a second working electrode in parallel to a first working electrode. In this configuration, one working electrode is sensitive to both hydrogen and carbon monoxide, and the second responds only to hydrogen. The difference between the two currents, properly scaled and calibrated gives a corrected carbon monoxide concentration. However, calibration is difficult and this four electrode system is also prone to drift.
The paper, “A sandwich electrode for multi-gas analysis: a prototype”, by C. E. W. Hahn et al, Br. J. Anaesth. (1982), 54, 681, discusses a system for simultaneously measuring two or more gases in a sandwich electrode configuration in which a first, external, metallised membrane simultaneously measures and filters out a first gas and a second, internal, cathode assays a second gas.
EP 1154267 A (Alphasense Ltd) discusses the alteration of properties, such as capacitance, of the electrochemical cell so that the additional working electrode current due to hydrogen is cancelled out by the current which flows between the working electrode and counter electrode to re-establish the potential difference between the working and reference electrodes when hydrogen reacts at the reference electrode.
EP 1154267 A (Alphasense Ltd) also discloses a four electrode electrochemical cell using a reference electrode in contact with interferent and a reference electrode located so as to not be affected by interferent.
U.S. Pat. No. 5,635,627 discloses a sensor in which the hydrogen cross sensitivity is reduced by providing two layers enriched by mercury and/or mercury ions, one on the surface of the working electrode and one on the surface of the reference electrode. It is known that mercury poisons the platinum black catalyst, reducing hydrogen sensitivity.
U.S. Pat. No. 4,681,115 (Drägerwerk Aktiengesellschaft) discloses a device having an additional electrode for sensing an external electrical potential, mounted in the outer surface region of a diffusion membrane.
U.S. Pat. No. 4,478,704 (Hitachi) uses a specified configuration of electrodes and electrolyte chamber to reduce hydrogen cross sensitivity.
U.S. Pat. No. 4,775,456 (Teledyne Industries, Inc) discusses an electrochemical gas analyser having a compensation electrode which provides a compensation signal related to the concentration of gas dissolved in the bulk of an electrolyte. This compensation signal is subtracted from the analyser output signal to give the resulting reading.
Also, some workers in the field move the bias voltage, to reduce hydrogen cross-sensitivity.
WO96/04550 (Huggenberger) uses a selective membrane and catalyst to eliminate a particular interfering gas, although we are not aware of a suitable membrane for use with hydrogen.
It is known that anodisation of a platinum working electrode leads to a permanent change in the response of the electrode to carbon monoxide and hydrogen. Anodisation reduces the cross sensitivity of a platinum working electrode, but also reduces the sensitivity of the electrode to carbon monoxide. As discussed above, those skilled in the art prefer to use as little catalyst as possible, partly for reasons of cost and partly because of the well known result that hydrogen cross sensitivity increases as the amount of catalyst is increased.
However, we have discovered that a platinum working electrode, previously subjected to an oxidation process, such as anodisation, does not suffer from increased hydrogen interference as the amount of catalyst is increased markedly, at least to the extent experienced with a non-oxidised platinum working electrode. This realisation enables us to provide a sensor having an amount of catalyst which, according to the generally understood theory, should not be capable of providing the low hydrogen cross sensitivity which results.
In this specification, the tern “activity capacity” is used to denote the ratio Io/Ie as measured by the following experiment, in which Ie is the working electrode current of an assembled sensor, having a diffusion restriction means, minus the background working electrode current (when no carbon monoxide is present) and Io is the working electrode current of a sensor having the diffusion restriction means removed so that it is open to excess gas sample, minus the background working electrode current.
Activity capacity is to be measured with the sensor connected to a potentiostat with a zero potential bias applied between the working and reference electrodes, in a gas mixture consisting of atmospheric air with 800 ppm carbon monoxide added thereto at a temperature of 5° C., at atmospheric pressure. Activity capacity decreases with reducing temperature, and so is tested near the lowest usage temperature.
The activity capacity can be seen as a measure of the amount of usable catalytic sites available as Io depends on the number of catalytic sites where carbon monoxide can react.
The term “cross sensitivity” is used in this specification to denote the extent to which a carbon monoxide sensor is affected by hydrogen in the following experiment.
Cross sensitivity is to be measured with the sensor connected to a potentiostat with a zero potential bias applied between the working and reference electrodes and exposed separately to two different gas mixtures: the first gas mixture consisting of 2.5% oxygen in nitrogen with 800 ppm carbon monoxide added thereto, at a temperature of 40° C., at atmospheric pressure. The second gas mixture consisting of 2.5% oxygen in nitrogen with 800 ppm carbon monoxide and 800 ppm hydrogen added thereto, also at a temperature of 40° C., at atmospheric pressure. Cross sensitivity increases with increasing temperatures and so is tested near the highest usage temperature.
The cross sensitivity is defined as the working electrode current when the sensor is exposed to the second gas minus the working electrode current when the sensor is exposed to the first gas, as a percentage of the working electrode current when the sensor is exposed to the second gas. (The background current when exposed to 2.5% oxygen in nitrogen is to be subtracted from each working electrode current prior to calculating cross sensitivity.)